Climate Change Could Alter the Human Male-Female Ratio

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Climate change could affect the ratio of human males to human
females that are born in some countries, a new study from Japan
suggests. The researchers found that male fetuses may be
particularly vulnerable to the effects of climate change.

Since the 1970s, temperature fluctuations from the norm have
become more common in Japan, and at the same time there has been
an increase in the deaths of male fetuses, relative to the number
of deaths of female fetuses in that country, according to the
study.

Over this period, the ratio of male to female babies born in the
country has been decreasing, meaning there have been fewer and
fewer male babies born relative to the number of female babies
born.

In the study, the researchers looked at monthly temperature data
gathered from 1968 to 2012 by the Japan Meteorological Agency and
also at data on fetal deaths and infants born during that time
from the Vital Statistics of Japan database. In recent years,
there have been nearly 90,000 newborns, and about 1,000 fetal
deaths recorded monthly in Japan. The researchers considered
fetal deaths to be those that were spontaneously aborted (or
miscarried) after 12 weeks of pregnancy.

The study also looked at two recent
extreme weather events in Japan — a very hot summer in 2010
and a very cold winter in 2011. During the hot summer —
which was the warmest in the country since 1898 —
there was an increase in the number of fetal deaths in
September of that year, and nine months later, there was a
decrease in the ratio of male to female babies born in the
country.

A similar phenomenon occurred the next year — during a very cold
winter in January 2011, there was an increase in fetal deaths,
and nine months later, there was a decrease in the number of male
babies born relative to female babies born in that country.
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These findings suggest that "the recent temperature fluctuations
in Japan seem to be linked to a lower male: female sex ratio of
newborn infants, partly via increased male fetal deaths," the
researchers wrote in the Sept. 14 issue of the journal Fertility
and Sterility.

However, the new study only found an association, and cannot
prove that the climate changes were responsible for the change in
sex ratio in Japan. Other factors, such as pollution and toxins
in the environment, may
affect sex ratios. But the researchers noted that the study
found a link between temperatures in a specific month, and the
sex ratios nine months later, suggesting temperature fluctuations
may play a role in recent declines in the country's sex ratio.

Previous studies on the effects of temperature changes on sex
ratios in human populations have been mixed. A 2008 study
published in the journal PNAS found a link between cold
temperatures and a decrease in the male-to-female ratio in
Scandinavian countries between 1865 and 1914, but also found a
link between warmer temperatures and an increase in the ratio.

In addition, two previous studies in Finland and New Zealand did
not find a link between those countries' yearly average
temperature and sex ratios of newborns from the mid-1800s to the
mid-2000s.

However, the researchers in the new study noted that neither New
Zealand nor Finland experience the same temperature extremes as
Japan, where summers can be very hot and winters very cold. Japan
has also experienced a greater increase in its average yearly
temperature in recent years compared with New Zealand and
Finland, the researchers said.

"It is of great interest to know whether any country observed a
similarly lowered sex ratio of newborn infants" during the recent
extreme weather events, the researchers said.

The reason why male babies may be more vulnerable to extreme
temperatures is not known. But it has been reported previously
that male embryos are affected negatively by stress factors such
as earthquakes or toxic agents, Fukuda said.

Some studies have also found that male births decline with
increasing geographical latitude (i.e., colder climates) in
Europe and Asia. The new findings warrant further investigation
of how stress factors like climate change may affect the ratio of
male to female infants born, the researchers said.

Some studies have suggested that recent changes in Japan's sex
ratio of births may be a result of the March 2011
earthquake and tsunami, which caused more than 18,000
deaths. But the researchers say this is not the case. Although
there was an overall reduction in the percentage of births in the
country nine months after the earthquake, there was no
significant decline in the sex ratio of births, the researchers
said.